In transmission and reception of electromagnetic radiation at rf, microwave, and millimeter wave frequencies, great efforts have been placed on the fabrication of high performance antennas which are manufacturable in high volume production while maintaining a cost which is acceptable in the rf, microwave and millimeter wave industries. One of the major areas in which cost can be checked is in the materials used in the fabrication of the array antenna. To this end, standard practice has been to use a teflon substrate as the dielectric material for the array antenna, the teflon substrate being disposed between the metal ground plane and the metal array. In such an antenna, a copper array is fabricated on the top surface of the teflon dielectric substrate, while a ground plane also of copper for performance purposes is disposed on the lower surface of the teflon substrate.
An alternative to the use of teflon as the substrate dielectric in array antenna applications is the use of a teflon composite having a glass mesh interspersed in the teflon material. The glass mesh in teflon has the advantage of providing structural stability and strength to the dielectric substrate and resultant array antenna. However, the material is intrinsically nonhomogeneous, and accordingly there are places where the differing dielectric constants of the differing materials result in variations in the impedance of the array antenna elements and transmission lines. Ultimately, particularly at narrow transmission-line widths, there are resulting impedance mismatch problems which have a direct impact on array performance. Accordingly, the teflon-glass composite material has been found to be an unattractive alternative to the teflon substrate in an array antenna.
The primary drawback to the use of teflon and teflon-glass substrates as the dielectric in the array antenna is that these materials are available at a relatively high cost, a cost level that is unacceptable for the wireless industry needs. Accordingly, while teflon and teflon-glass composites exhibit acceptable performance for array antennas in the wireless industry, better performance is desired, as well as a reduced cost of manufacture.
An alternative approach to the use of teflon as the substrate for the array antenna is a material having the trade name TPX, and is manufactured by Matsui Path Tek. The chemical composition of TPX is polymethylpentene or PMP. PMP has the same dielectric constant as teflon, and similar or better loss tangent as teflon. Accordingly, PMP appears to present an attractive alternative in that it is fungible with teflon from a performance standpoint, however, is available at a much lower cost. Both teflon and PMP are relatively low permittivity (.epsilon.) materials and their use in array antennas as the dielectric substrate enables the reduction of surface wave effects. Surface wave effects, which are readily understood by one of ordinary skill in the art through the analysis of boundary value conditions in electromagnetic theory, result in losses due to energy trapped in the dielectric. As stated, the use of low permittivity (.epsilon.) dielectric substrates such as teflon and PMP reduce the undesired surface or evanescent wave effects.
Accordingly, PMP has the desirable characteristics of reduced surface wave effects, similar or better loss tangent characteristics as teflon and is available at a substantially reduced cost when compared to teflon. However, one drawback that is presented with the substitution of PMP for teflon as the substrate dielectric for an array antenna is that the standard technique for adhering copper to teflon and the subsequent etching to form the metal pattern of the array antenna will not work when PMP is used as the substrate. Furthermore, while low dielectric constant materials have the attendant benefit of reduction of surface or evanescent wave effects, these materials are susceptible to the ill effects of undesired radiation at feeder discontinuities in the array transmission lines. These feeder discontinuities result in losses due to noncoherant radiation having various polarizations with the overall result that increased antenna losses are realized. The undesired radiation resulting from discontinuities in the feeder line to the individual antenna array patches are shown more clearly in FIG. 1. To this end, at each discontinuity of the exemplary feeder line, undesired radiation which is a direct result of the discontinuities of the transmission line on the low permittivity material, is evident at the discontinuity points as shown. One alternative would be to use a higher dielectric material as the substrate. This could possibly reduce the ill effects of discontinuity radiation loss, however most of the commercially available materials have higher dielectric loss tangent values that would result in unacceptable levels of loss in the antenna.
Accordingly, what is required is a low cost transmission line for an array antenna which has the attendant advantage of reduced surface or evanescent wave effects, and thereby a reduction in losses associated therewith, without the disadvantages of feeder discontinuity losses.